1. Technical Field
The present invention relates to a driving assist controller for a vehicle that operates an electric power steering motor to enable the vehicle to travel along a set target course.
2. Related Art
Various driving assist controllers for a vehicle that assist driving to enable the vehicle to run along a set target course have recently been developed with the object of decreasing the number of traffic accidents and reduce a load on a driver. A forward gaze point model simulating the driver's operations has been widely known as a technique for calculating a target steering angle for causing the vehicle to run along the target course. For example, Japanese Unexamined Patent Application Publication (JP-A) No. 2005-170327 discloses an automatic steering control device for a vehicle in which a forward gaze point that is set in front of a vehicle equipped the device is set farther from the vehicle when the vehicle speed increases, and a steering amount is calculated on the basis of a difference between the forward gaze point and a reference path constituting the running target path. In this automatic steering control device, the difference based on the forward gaze point that has been set on the curve ahead is corrected to be brought closer to the difference based on the forward gaze point that is set when the vehicle enters the curve.
However, when the target steering angle is set according to the transverse deviation from the target course in the forward gaze point, such as disclosed in JP-A No. 2005-170327, where a distance from the present vehicle position to the forward gaze point is small, or a prediction time (time till the vehicle reaches the forward gaze point) is short, the proximity of the vehicle is viewed, the operation allowing the vehicle to track rapidly the target course is performed, and the target course tracking ability is improved, but nervous and unstable target steering angle or vehicle behavior, which is focused only on the present transverse deviation, is realized. Meanwhile, where the prediction time is extended, the operation that may return the vehicle to the target course after the prediction time is performed and the target course tracking ability is slowed down, but the prediction control is performed that takes into account not only the transverse deviation from the target course, but also the orientation of the vehicle with respect to the target course, and the change ratio thereof (yaw rate), and stable target steering angle and vehicle behavior are obtained. It follows from the above, that when the driving assist control is executed with the forward gaze point model, the prediction time should be adequately set such as to balance the rapid target course tracking ability with the stability of vehicle behavior. Setting the forward gaze point according to the vehicle speed, as disclosed in JP-A No. 2005-170327, is equivalent to maintaining the prediction time. The resultant problem is that although the vehicle behavior is stabilized according to the increase in vehicle speed, the ability to track the target course at a high speed is slowed down. Further, a function of changing the forward gaze point according to the road radius ahead is also disclosed as a measure alleviating the concern that when the vehicle enters a curve, the forward gaze point becomes too far on the curve and the control that takes no account of the subsequent deceleration operation is performed. However, this is merely the correction assuming the vehicle speed at the time when the vehicle enters the curve, and basically equivalent to setting the distance to the forward gaze point that is sensitive to the vehicle speed.